The present invention relates to a closed deck type cylinder block and a method for producing the block, and more particularly to the closed deck type cylinder block having a reduced open area ratio of an opening of a water jacket portion opened at a top deck side of the cylinder block, and method for producing such cylinder block with the reduced open area ratio.
In the cylinder block for a vehicle, a water jacket is formed surrounding a plurality of cylinders arranged juxtaposedly. The water jacket has an upper open end opening at the top deck surface of the cylinder block. In a closed deck type cylinder block, the upper open end of the water jacket is partly covered with bridges, and in an open deck type cylinder block, no bridge is provided at the opening. Recently, attention is drawn to the closed deck type cylinder block with a reduced opening area ratio at the top deck surface because of the requirement of high output and noise reduction of an engine. Such cylinder block can provide high rigidity and reduced vibration, which improves engine performance and meets with environmental demand.
According to a conventional method for die casting the cylinder block of this type, a coated core for forming a water jacket portion (hereinafter simply referred to as xe2x80x9cwater jacket corexe2x80x9d) is assembled in a metal mold, and a molten light metal such as molten aluminum alloy is injected into the metal mold at low speed with a plunger speed of not more than 0.5m/sec. to provide a laminar flow of the molten metal. The casted product (cylinder block) is then subjected to shaking treatment and if necessary, heat treatment, to remove wasted core sands through the opening at the top deck surface. The resultant product provides high rigidity and does not involve blisters which may be formed due to heat treatment (for example, heat treatment to the casted product containing the core) to the casted product produced through high speed injection (turbulent flow).
After casting, shaking treatment or heat treatment is performed to fractionate the wasted core sand so as to discharge the wasted core sand through the opening of the water jacket portion. For example, if a core contains an easily thermaly decomposable and curable organic binder, which comprises as the main component, polyfunctional acrylamide having at least two ethylenically unsaturated groups in one molecule as disclosed in international publication No. WO 90/02007, the wasted core sand is discharged by way of the shaking treatment.
In the closed deck type cylinder block, the increase in occupying ratio of the bridge portions with respect to the opening of the water jacket implies reduction in an area ratio of the opening with respect to a projection area of the water jacket portion at the top deck side, the projection being made in an axial direction of the cylinders. Therefore, the waste sand discharge through the opening becomes more difficult. In other words, the requirement of reduction in the opening area ratio is in direct conflict with the requirement of enhancing the removal of the waste sand through the opening. Consequently, optimum area ratio of the opening of the water jacket must be selected in the closed deck type cylinder block.
Further, if a difficulty thermally decomposable phenolic resin is used as a curable organic binder in the production of the water jacket core, both shaking and heat treatment are required, which incurs greater energy cost and labor. Furthermore, in case of the water jacket core produced by the materials described in international publication No. WO 90/02007, great amount of heat from the molten metal is transmitted to the water jacket core if the sand/metal ratio (weight ratio of the core to the molten metal) is low. Therefore, in accordance with the progress of thermal decomposition, tar is generated and the viscosity of the tar will degrade the removability of the waste sand from the water jacket portion.
It is therefore an object of the present invention to provide a closed deck type cylinder block providing an optimum area ratio of the opening of the water jacket portion with respect to the projection area of the water jacket portion at the top deck side.
Another object of the present invention is to provide a method for producing a closed deck type cylinder block capable of providing an extremely low area ratio of opening ranging from 10 to 30% of the projection area of the water jacket at the top deck side in a die-casting method employing a coated water jacket core, yet capable of providing excellent waste sand discharging performance while reducing energy cost and labor for the production.
These and other objects of the present invention will be attained by a closed deck type cylinder block having a top deck surface and formed with a plurality of cylinders juxtaposedly arranged to each other and a water jacket portion surrounding the plurality of cylinders, the water jacket portion having an opening opened at the top deck surface of the cylinder block, and the opening being partly closed by bridge portions. The opening has an area ratio of from 10 to 30% of a projection area of the water jacket portion at the top deck surface.
By setting the area ratio of opening not less than 10%, a resultant cylinder block can provide a smooth discharge of the waste sand converted from the water jacket core through the opening, and inadvertent increase in hydraulic pressure of a coolant in the water jacket can be avoided, thereby avoiding inadvertent breakdown of a feed pump for feeding the coolant. Further, by setting the area ratio of opening not more than 30%, sufficient mechanical strength of the cylinder block can be provided to enhance engine performance and to enhance silence of the operating engine.
In another aspect of the invention, there is provided a method for producing the above described closed deck type cylinder block. The method includes the steps of: molding a water jacket core with a mold material with the free flowability comprising a refractory aggregate, a curable organic binder containing as a requisite component polyfunctional acrylamide having at least two ethylenically unsaturated groups in one molecule, and at least one metal oxide selected from the group consisting of iron oxide and copper oxide; forming a coating layer over the molded water jacket core;
Setting the coated water jacket core in a metal mold and injecting a molten metal into the metal mold to produce a cylinder block product by die-casting; and discharging waste sand converted from the water jacket core from the cylinder block product only by shaking the cylinder block product.
Because the water jacket core is molded with using the curable organic binder containing polyfunctional acrylamide as a requisite component and the specific metal oxide, and because the molded core is subjected to coating, a complete discharge of the waste sand after casting can be performed only by shaking the cylinder block product, even if the resultant closed deck type cylinder block has a small opening area ratio such as from 10 to 30% of the projection area of the water jacket portion at the top deck surface. Particularly, in case of the co-use of the curable organic binder containing polyfunctional acrylamide and iron oxide or a mixture of iron oxide and copper oxide, tar generated at the thermal decomposition of the curable organic binder can be reduced. Accordingly, waste sand discharging performance can further be improved after casting, and only shaking process is required for completely discharging the waste sand from the cylinder block product. Thus, entire processing period can be reduced, and productivity can be enhanced.